SIP Codec Negotiation

When an inbound SIP call is received, the 2020 IMG will negotiate what
Codec to use when connecting the call. When negotiating a Codec selection
the 2020 IMG must first know whether to use the Codec selections of the
remote SIP gateway (Selections are in the SIP INVITE message) or to use
the Codec selections from the local 2020 IMG. Below is a screen capture
of the SIP Profile Object Pane in ClientView. Note that either Local or
Remote can be selected from a drop down menu in the Codec Priority Field.

Note: The default
Codec Priority is "Local". To change this you must create a
new SIP profile. Once the new profile is created then the drop down menu
will be accessible. Once the SIP Profile ID is updated to something other
than 0 (Default) you will be able to edit the fields in this Pane.

Codec Priority

Local: (Default) If Local is
selected then the 2020 IMG will look at its own Codec Priority List under
the IP Bearer Profile Object Pane and select from that list the highest
priority Codec that matches a Codec from the list of Codecs in the SDP
Offer message from the remote Gateway. (Overlap Condition) This will be
the Codec used. This ensures the 2020 IMG or local end decides which codec
to use.

Remote: If Remote is selected
then the 2020 IMG will look at the incoming Codec Selections in the SDP
Offer message from the remote gateway and match the highest priority Codec
from that list with one of the selections from the IP Bearer Profile object
pane in ClientView. This will ensure the remote end is deciding which
Codec to use.

Example1:

Consider the following media list offering from a remote Gateway. Highest
priority is #1

G.711 ulaw

G.711 alaw

G.723

m=audio 5108 RTP/AVP 0 8 97

If the SIP SDP Profile Object Pane has Local
set in the Codec Priority field and the following Codecs below are configured
under the IP Bearer Profiles Pane in ClientView(1 is highest priority)
. The negotiated Codec would be G.723. G.711 alaw is the highest priority
vocoder offered from the remote gateway, but the highest priority vocoder
in the 2020 IMG list that is also offered from the remote side is G.723.
The 2020 IMG will give preference to its ordered list of vocoders and
will respond offering only G.723.

G.729

G.723

AMR

G.711 alaw

It is important to note that when the Codec Priority is set to Local
the 2020 IMG gives priority to the list of codecs defined in the IP Bearer
Profile object and does not respond with all vocoders in the overlap group
.

Note: The overlap
of vocoders includes G.711 alaw and G.723.

Example2:

Consider the following media list offering from the remote Gateway.
Highest priority is #1

G.711 ulaw

G.711 alaw

AMR:

m=audio 5108 RTP/AVP 0 8 97

If the SIP SDP Profile Object Pane has Remote
set in the Codec Priority field and the following Codecs below are configured
under the IP Bearer Profiles Pane in ClientView. (1 is highest priority)
The negotiated Codec would be G.711 alaw. G.711 alaw is the highest priority
vocoder offered from the remote gateway that overlaps with the Codec Priority
List configured in the 2020 IMG. The 2020 IMG will give preference to
the remote Gateways ordered list of vocoders and will respond offering
only G.711 alaw.

G.729

G.723

AMR

G.711 alaw

It is important to note the 2020 IMG gives priority to the list of codecs
defined in the remote Gateways Codec Priority List which is sent in the
INVITE message.

Note: The overlap
of vocoders includes G.711 alaw and AMR.

Payload Size Priority

Preferred Payload Size, Maximum Payload Size, Minimum Payload Size

The Payload Size is negotiated with the remote gateway but does not
negotiate using an ordered list. Below explains how the Payload size is
negotiated.

If the remote gateway sends a preferred payload in the incoming
SDP Offer (ptime) and the payload size is within the 2020 IMG's preferred
payload Min/Max settings in the Supported Vocoders Object pane then
the 2020 IMG will negotiate to that payload size. See screen capture
below of the SIP Profile Object Pane

If the remote gateway does not send a preferred payload size or the payload size sent is not
within our Min/Max range the 2020 IMG will use its own Preferred Payload
size.